1. Field of the Invention
The present invention relates generally to an induction system for an internal combustion engine having multiple inlet valves for each combustion chamber there of and more specifically to such an induction system which reduces the amount of residual gas retained in the combustion chamber and induction conduiting immediate upstream of the combustion chamber, when the engine is operating under low speed/load conditions such as idling, and thus improves the combustion process under such conditions.
2. Description of the Prior Art
FIGS. 1 to 3 show an induction system proposed in Japanese Patent Application First Publication No. 47-31724. In this arrangement the induction system includes a so called "low speed" induction passage 1 and a "high speed" induction passage 2 which branches off from the former. Poppet valves 3, 4 control communication between the combustion chamber 5 and the respective induction conduits 1, 2. A butterfly type flow control valve 6 is disposed in the high speed induction passage 2 and downstream of the engine throttle valve 7. This valve is arranged to prevent the flow of air through the "high speed" passage 2 during the aformentioned low speed/load mode of engine operation.
Poppet valve 4 is arranged to be controlled in a timed relationship with the exhaust valve 8 so as to provide a large valve overlap (see solid line trace in FIG. 2). This of course improves engine respiration during high speed. On the other hand, poppet valve 3 is arranged to be opened and close with a relatively small overlap which is suited to low-intermediate speed/load operation such as idling (See chain line trace in FIG. 2).
However, with this type of arrangement a considerable volume tends to be defined in the high speed passage 2 between the flow control valve 6 and the poppet valve 4 (for example 15-20% of the cylinder clearance with the piston at TDC) when the engine is idling and/or operating under low speed load condition. Further, a relatively high vacuum (e.g. 400 mmHg) develops in this section of the "high speed" induction conduit under such conditions. Accordingly, as the butterfly valve 6 is closed at this time so as to effectively render the high speed inlet valve inoperative, when the latter in fact lifts during idling, the exhaust gases contained in the combustion chamber at the beginning of the exhaust phase are sucked into and mix with the air or air-fuel mixture trapped in the section of high speed induction conduit between the valves. This undesirably causes an increase in the amount of exhaust gas (viz., residual gas) which fails to be discharged during the exhaust phase and markedly degrades the combustion process under such conditions.